The answer is WRECK.
As a noun, 'decay' is the state of gradual damage or decline in strength, soundness, or prosperity, that is to say, of becoming worse. Similar to the noun 'wreck' that is the state of being damaged, disabled, in ruin or dilapidation.
The other words do not support the meaning of 'decay', because 'colossal' refers to something extremely big, 'bare' is something/someone without any clothes or not covered by anything, and 'round' is something shaped like a ball or circle, or curved.